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Old 07-05-2021, 10:15 AM   #1
bigdog
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Default Oil type viscosity / Regular or Synthetic ?

I will be changing my oil & filter in my 2000 4.3L Mercuiser V6 this week.

I don't have an owner's manual, but what I have been able to research the manufacturer recommends 20Wx40 Synthetic oil.

I am the second owner, and the original owner always used conventional straight 10Wx40.

Questions:
1) Is there a major difference between 10W and 20W viscosity ?
2) Is there an advantage using 20W oil ?
3) Conventional oil vs Synthetic ? Is there a real advantage ? I know conventional oil contains carbon, which can leave deposits on valves and pistons, which can shorten life, but using in a boat where one only puts few hours use per year, would 'Syn' really make any difference ?
4) Thoughts regarding 'synthetic blend' oil ?

Feedback appreciated....

Thanks !
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Old 07-05-2021, 11:06 AM   #2
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If the manufacturer says use 20W40 synthetic, use 20W40 synthetic. It costs you all of 20 bucks more for your once a season oil change and trying to save that doesn't make a lot of sense.

Yes, there's a difference between 10W and 20W. Basically those numbers mean a 10W40 oil flows like a 10 weight and protects like a 40 weight, not that I'm a chemist to understand what's really going on. Running a lighter than recommended viscosity can result in lower oil pressure, particularly at idle once hot.

Wrong about the carbon, conventional oil doesn't contain it and leave deposits. It's just a different chemistry. Listen, you could run conventional 10W40 for 20 years and not have a problem. But why chance it? Run what the manufacturer recommends and you are taking the fewest chances on the life of your engine.
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Old 07-05-2021, 11:54 AM   #3
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Synthetic oil processes have been around for some time. A base oil material derived from crude oil is subjected to a process that breaks down and reforms those hydrocarbon compounds into more uniform oil compounds, with better lubricating quality and more resistant to breaking down under the high temperatures found in an engine. But both regular and synthetic oils are fundamentally hydrocarbon compounds, so they do contain carbon, locked up chemically within the compounds. However, the synthetic oils are more chemically stable than regular oils. More info on the subject can be found here: https://en.wikipedia.org/wiki/Synthetic_oil
and elsewhere. A simple search tells you far more than you want to know.
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Old 07-05-2021, 02:42 PM   #4
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Thanks DickR for the education about this...

I haven't heard or actually understand what 'synthetic blend is' ?
Half and half ?
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Old 07-05-2021, 02:53 PM   #5
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What brand of full synthetic oil do the boaters here like?

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Old 07-05-2021, 03:08 PM   #6
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For those of you who subscribe to Classic Boating, there is a good article in the July/August issue entitled How Engines Work part II. Part of the article discusses oils. The "W" stands for winter which means it will hold it's viscosity at 0 degrees F. Otherwise the oil will hold it's viscosity at 210 degrees F. The higher the rating ie. 30,40,50 is for high compression engines. The oil temp for marine engines can go up to 240-250F which is why multigrade viscosity is advised. synthetic oils contain only 1 or 2 compounds and are derived from the liquefaction of natural gas. Synthetic oils will not breakdown as conventional oil does because of the temperature.
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Old 07-05-2021, 03:37 PM   #7
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Quote:
Originally Posted by bigdog View Post
Thanks DickR for the education about this...

I haven't heard or actually understand what 'synthetic blend is' ?
Half and half ?
Yup, though not necessarily 50/50. Think of it as a conventional oil base with a synthetic additive package.

Kind of like if you don't want to drink whole milk, but you don't like skim, you can go with 1 or 2% and get almost the whole milk taste with almost the skim milk reduced fat.
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Old 07-05-2021, 04:45 PM   #8
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Yeah, I like 2%

Going to purchase and fill with good synthetic brand:
Castrol, Quaker, Pennzoil, Valvoline, etc
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Old 07-05-2021, 07:33 PM   #9
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I don't own a Mercruiser so haven't ever bought it, but don't rule out buying the Mercury Quicksilver oil they recommend. I don't know the price, obviously can get it at a boat dealer but I think also at Walmart and maybe some auto parts stores.

Out of curiosity I took a look at Mercury's oil site, they say it's a 25-40 synthetic blend. So there you go, maybe Mercury broke their own rules on the full synthetic recommendation, not sure. Interesting.
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Old 07-08-2021, 09:24 AM   #10
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Your 4.3 is an automotive-based Chevy without any smog controls like a catalytic converter. If you want the best protection, use diesel oil instead of gasoline powered auto or marine oil. Non-diesel oils are formulated with far less Zinc Dialkyl Dithiophosphates or ZDDP due to the fact that ZDDP damages gasoline engine catalytic converters if it makes it into the combustion process. Since you don't have a catalytic converter, you can get the advantage of a lot of ZDDP (which is fantastic for preventing engine wear), so you might as well do so. The best part of the whole deal is that diesel oil is typically a lot cheaper than gasoline powered auto oil.

As far as viscosity goes, the number preceding the W is irrelevant as along as it is lower than or equal to what the manufacturer recommends. 5W-40 protects at high temperatures just as well as 25W-40. The 40 is the key parameter.

Unless you are putting more than 200 hours on your oil between changes, or you are using the boat in weather well below freezing, there's no advantage to synthetic or synthetic blend oil.

I'm a huge fan of Shell Rotella T4 15W-40 in applications like yours, I buy it in 5 gallon buckets and it's great stuff for very little money. If you really want synthetic, T6 5W-40 is also available.

For those that have marine engines with catalytic converters, you really need to use oil that meets or exceeds the recommendations of the engine builder. Don't use diesel oil.
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Old 07-08-2021, 04:17 PM   #11
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Default Rotella

I agree with DAVE R. I use Rotella exclusively in my 454 Mercruiser. 1200+ hours before a rebuild. 15w-40.
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Old 07-08-2021, 04:34 PM   #12
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I concur with BroadHopper and DaveR, Shell Rotella T4 15W-40, what I've researched is a great and 'under-rated' oil.
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Old 07-08-2021, 04:50 PM   #13
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Quote:
Originally Posted by BroadHopper View Post
I agree with DAVE R. I use Rotella exclusively in my 454 Mercruiser. 1200+ hours before a rebuild. 15w-40.
1200+ hours!!!

You deserve the boat owner's self discipline award.

Most owners move on to the next boat much sooner than that.

Last edited by 8gv; 07-08-2021 at 11:18 PM.
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Old 07-08-2021, 08:43 PM   #14
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Quote:
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I'm a huge fan of Shell Rotella T4 15W-40 in applications like yours, I buy it in 5 gallon buckets and it's great stuff for very little money. If you really want synthetic, T6 5W-40 is also available.
BTW there is a T6 15W40 now too. I had read about it, then picked up a jug by accident and didn't notice until I went to use it. I run 5W40 T6 in my Duramax.
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Old 07-09-2021, 09:56 AM   #15
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Quote:
Originally Posted by LikeLakes View Post
BTW there is a T6 15W40 now too. I had read about it, then picked up a jug by accident and didn't notice until I went to use it. I run 5W40 T6 in my Duramax.
I ran 5W40 T6 in my Duramax when I had it, too. Great oil and easy Winter starts. I run T4 in my boat's Caterpillar engines though. They each take 4 gallons, they don't run in the Winter and I don't do extended change intervals, so T4 is the best choice for me. I love that Walmart always has it in stock and they take 5 gallons at a time for recycling.
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Old 07-09-2021, 10:45 AM   #16
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Motor oil is literally the only reason I ever go to Walmart! Agree T6 is a great oil for diesels in New England.

I'm totally on board with those that know more than I do about oil for the Merc 4.3. I answered the OP the way I did because when in doubt, do what the manufacturer recommends is a good plan, with a few exceptions here and there.
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Old 07-14-2021, 03:49 PM   #17
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Having an issue finding Shell Rotella 20Wx40 T4 at Walmart, they only stock 15W, would this be OK?

Is there that much difference?
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Old 07-14-2021, 08:20 PM   #18
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Really good info from both Dick R and Likelakes. The second number in the designation is a bit more critical than the first as it is the base weight and viscosity when the oil is at running temperature. The first number is the cold viscosity and while it is best to be close it isn't as critical. Due to the cold viscosity additives the 15W will crank ever so slightly better than the 20W but it really affect your performance or longevity at all.
I used to run only Rotella in all of my engines out of habit because it was one of the last blends to have zinc additive but now use Walmart's Supertech brand full synthetic. Zinc is only really important when you have an engine with solid lifter cam and unless you have an old muscle car you needed be concerned about it. I do run full synthetic in everything, including my Duramax, Kubota, and vehicles simply because it cranks so much better when cold and isn't really that much more costly.
I have a mini-excavator that would crank but not start if it was below 30 ...until I changed both the engine and hydraulic oils to full synthetic. Now I can get it to start and move it out of the way even if it is only 20' out. pretty convincing. Second, I used to have a Vmax motorbike that had a common sump for both the engine and tranny and once I switched to full synthetic I found that it shifted much easier when the oil wasn't fully warmed up.
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Old 07-14-2021, 10:26 PM   #19
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Quote:
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Zinc is only really important when you have an engine with solid cam rollers
You mean flat tappet engine, like a 351 Ford for example. Not meaning to nit-pick.
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Old 07-17-2021, 05:03 PM   #20
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Quote:
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Having an issue finding Shell Rotella 20Wx40 T4 at Walmart, they only stock 15W, would this be OK?

Is there that much difference?
Yup, it's just fine.
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